1. Field of the Invention
The present invention relates to a method of bonding base materials and a method of manufacturing an image display apparatus, and more particularly to a method of bonding members constituting an envelope of the image display apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2000-106108 discloses a manufacturing method of a cathode ray tube, which includes a step of glass-bonding (frit-sealing) a face plate having a phosphor layer of displaying an image in response to irradiation of electrons emitted from a cathode ray tube and a funnel cone portion of acting as an outer container of the cathode ray tube. Since a thermal expansion coefficient of the funnel cone portion is different from a thermal expansion coefficient of the face plate by at least 10×10−7/° C. or more, there is a possibility that breakage or crack occurs in the funnel cone portion and/or the face plate due to thermal contraction of frit glass at the time of the glass bonding. According to Japanese Patent Application Laid-Open No. 2000-106108, the thermal expansion coefficient of the frit glass is made different for each of parts so as to prevent the occurrence of breakage or crack. For example, Japanese Patent Application Laid-Open No. 2000-106108 describes a method of changing stepwise the thermal expansion coefficient of the frit glass between the face plate and the funnel cone portion, and a method of constituting the frit glass as a laminated body of plural different kinds of frit glasses of which the thermal expansion coefficients are mutually different.
Japanese Patent Application Laid-Open No. 2008-517446 discloses an airtight sealing method of an organic light-emitting diode display. In this method, it is possible to airtightly seal a cover plate and a substrate with each other by irradiating a laser beam to a frit provided on the cover plate and thus melting the frit. A region including an electrode and a region not including an electrode exist along a sealing line on the substrate. For this reason, it is possible to uniformly heat the frit by properly changing moving speed and/or power of the laser beam along the sealing line.
As described in Japanese Patent Application Laid-Open No. 2000-106108, when the thermal expansion coefficient of the pair of the base materials to be bonded is large, a problem of occurrence of the breakage or the crack in the base material is actualized. In other words, when a difference between the thermal expansion coefficients of the base materials constituting the pair is below 10×10−7/° C. being a rough standard indicated in Japanese Patent Application Laid-Open No. 2000-106108, the problem of occurrence of the breakage or the crack in the base material is not so actualized. However, even when the difference of the thermal expansion coefficients is small and thus the breakage or the crack of the base material does not occur, there is a possibility that the base material is warped due to the difference between the thermal expansion coefficients.
The above-described problem will be concretely described with reference to FIGS. 5A to 5C. FIG. 5A illustrates a flat plate, a frame member and a bonding material which bonds the flat plate and the frame member to each other, and FIG. 5B is the cross section diagram which is viewed along the 5B-5B line in FIG. 5A. In the drawings, a bonding material 103 extends like a frame along a frame member 102 between a flat plate 101 and the frame member 102. If a laser beam is irradiated from the side of the frame member 102 or the side of the flat plate 101 to the bonding material 103, the bonding material 103 is melted and then hardened, whereby the flat plate 101 and the frame member 102 is bonded to each other. Since the irradiated laser beam is focused on the bonding material 103, large heat energy is applied to the bonding material 103, whereby the bonding material 103 reaches a high temperature. Further, the applied heat energy is transmitted also to the flat plate 101 and the frame member 102, thereby increasing temperatures of the flat plate 101 and the frame member 102. Each of the flat plate 101 and the frame member 102 thermally expands according to its thermal expansion coefficient. Since the bonding material 103 is being melted, the bonding material 103 is deformed according to thermal deformations of the flat plate 101 and the frame member 102. For this reason, since the flat plate 101 and the frame member 102 are not held by the bonding material 103, a stress is hardly generated on the flat plate 101 and the frame member 102. After then, although they are cooled, a stress is hardly generated on the flat plate 101 and the frame member 102 while the bonding material 103 is being melted because of the same reason as above. However, if the bonding material 103 begins to harden, the flat plate 101 and the frame member 102 are held by the bonding material 103. Then, the flat plate 101 and the frame member 102 are cooled down in this state, whereby the flat plate 101 and the frame member 102 begin to contract as indicated by the arrows in FIG. 5C. The thermal contraction at this time is thermal contraction so as to reduce the length of the side of each of the flat plate 101 and the frame member 102 while maintaining a similar figure of each of the flat plate 101 and the frame member 102. Here, thermal contraction amounts of the flat plate 101 and the frame member 102 are different from each other due to a difference of thermal expansion coefficients and a difference of temperature drop amounts between the flat plate 101 and the frame member 102. Thus, as illustrated in FIG. 5C, a warp occurs in the assembled body, in which the flat plate 101 and the frame member 102 have been bonded to each other by the bonding material 103, due to the difference between the thermal contraction amounts of the flat plate 101 and the frame member 102.